The fmt library happily accepts std::string without
any conversions.

Fixes multiple copyright messages in the doxygen docu.

ag '\\file' -L --ignore=ThirdParty *Lib |xargs  sed -i 's/\(.*\)\\copyright/\1\\file\n\1\\copyright/'

Including
 - usage of static_cast instead of c-style cast,
 - single variable declaration per line,
 - removing const from return types
 - omit unnecessary string to char* conversions,

The used clang-tidy checks are:
google-readability-casting,
readability-const-return-type,
readability-container-size-empty,
readability-delete-null-pointer,
readability-deleted-default,
readability-else-after-return,
readability-function-size,
readability-identifier-naming,
readability-inconsistent-declaration-parameter-name,
readability-isolate-declaration,
readability-misleading-indentation,
readability-misplaced-array-index,
readability-named-parameter,
readability-redundant-*,
readability-simplify-*,
readability-static-accessed-through-instance,
readability-static-definition-in-anonymous-namespace,
readability-string-compare,
readability-uniqueptr-delete-release

See previous commit for details.

The output and project folder are needed for
reading the balance mesh and writing the results
of the balance calculation.

Clang format run on near-the-header lines.

Happy New Year

 in creating of the instances of classes of coupled processes.